1. Field of the Invention
The present invention relates to an optical wavelength converting element which utilizes a Secondary Harmonic Generation (SHG) based on a Quasi-Phase Matching (QPM) and is so-called as a QPM-SHG element, and in particular to a process for forming polarization inversion layers with a QPM structure on a substrate made of a ferroelectric crystal used for the QPM-SHG element.
2. Description of the Related Art
The QPM-SHG element comprises an extended core or three-dimensional waveguide made of a ferroelectric body, a clad with a low index of refraction surrounding the waveguide and a periodic domain inversion structure along the extending direction of the waveguide. The QPM-SHG element generates a secondary harmonic of a half wave .lambda./2 for a fundamental wave .lambda. of an input light injected to the waveguide under the conditions of quasi-phase matching (QPM). In the QPM-SHG element as shown in FIG. 1, a substrate 1 of a ferroelectric crystal has a plurality of polarization inversion layers 3 disposed periodically along a three-dimensional waveguide 2 in which the fundamental wave is guided. The polarization of polarization inversion layers 3 alternately reverse side by side in the extending direction of the waveguide as the polarization directions are indicated by an arrowhead broken line in the figure. The fundamental wave is injected to an input end surface of the waveguide 2 and propagates across the polarization inversion layers 3, so that the secondary harmonic appears as an output from the other end with the fundamental wave. Generally, the second harmonics output has such a property that as this output propagates, it periodically reaches the peak and trough levels every coherence length. Quasi-phase matching is a matching method which utilizes this property to alternately invert the sign of a polarization wave that is generated every coherence length (periodic domain inversion structure), and adds the outputs of the second harmonics to increase the output.
For the ferroelectric crystal substrate 1 of the QPM-SHG element, crystals of lithium niobate (LiNbO.sub.3) having a high nonlinear optical coefficient (referring as LN hereinafter) and lithium tantalate (LiTaO.sub.3) insusceptible to optical damage (referring as LT hereinafter) are used as crystal substrates. The Z-axis or C-axis direction coincides with the polarization direction in LN and LT crystals. The LN and LT crystals have domain inversion characteristics by which the inversion of polarization in the direction of the Z axis of the crystal or the inversion of the polarization domain is likely to occur due to an external factor such as an impurity, distortion stress, heat, or an electric field. As shown in FIG. 1, a Z-cut substrate 1 of LN or LT crystal (substrate having, as a major surface, a Z cut face z containing the X-Y axes with the Z axis of the crystal as a normal line in FIG. 1) is mainly used as a wavelength converting element. There is a method of forming a periodic domain inversion structure or polarization inversion layers along an optical waveguide, for example, a heat treatment at near the Curie point, in which the proton exchange is conducted on the major surface of Z-cut substrate of a LN or LT crystal by application of pyrophosphoric acid and then the resultant substrate is subjected to heat treatment at near the Curie point. In this manner, a three-dimensional waveguide and polarization inversion layers are formed on the Z-cut surface of the substrate of a LN or LT crystal.
Generally, the conventional proton exchange method is performed on the Z-cut surface of the LN or LT crystal substrate, because the inversion of polarization occur easily on the Z-cut surface and further both the X cut face (x in FIG. 1) containing the Z-Y axes with the X axis of the crystal as a normal line and the Y cut face (y in FIG. 1) containing the Z-X axes with the Y axis of the crystal as a normal line are corroded by the proton exchange. It is therefore considered that the X-cut substrate having, as a major surface, an X cut face containing the Z-Y axes with the X axis of the crystal as a normal line and a Y-cut substrate having, as a major surface, an X cut face containing the Z-X axes with the Y axis of the crystal as a normal line are improper for forming proton exchange layers, due to proton-exchange oriented corrosion.
On the other hand, a laser light can not be directly coupled to the end surface of the waveguide of the QPM-SMG using the Z-cut substrate because of a TM mode optically coupling. When the QPM-SHG element is used for an optical pickup device, an optical system such as a mode converter is required between the QPM-SHG element and a laser light source. As a result the optical pickup device is inevitably maximized.
To avoid the maximized device, the use of an X or Y-cut LN or LT substrate for the QPM-SHG element is attempted. For instance, there is a process for forming a proton exchange layer with application of an electric field in which a comb-shaped electrode is provided on a cut surface parallel to the Z-axis containing the Z-axis of an X or Y-cut LN or LT substrate and then, an electric field is applied to the substrate through the comb-shaped electrode so that periodic polarization inversion layers are fabricated. Since the teeth of the comb-shaped electrode are connected to each other, the conductance difference of the teeth give an unwanted influence to micro portion of the crystal. Therefore the production of uniform polarization inversion layers is difficult. In addition, an electric field with a high intensity 1 KV/mm or more is required since the polarization inversion is performed under the condition of the Curie point or less. In this case a destroying of crystal may occur due to the application of a high intensity electric field. Since the comb-shaped electrode is flat on the cut surface parallel to the Z-axis (an X or Y-cut surface), it is difficult to form deep polarization inversion layers from the cut surface. Shallow polarization inversion layers superpose and intersect an optical waveguide but the overlap portions is very small. As a result, the converting efficiency from a fundamental wave to a secondary haramonic is not improved.
To form deep polarization inversion layers from the cut surface, there is a process of forming polarization inversion layers which, as shown in FIG. 2, utilizing an internal electric field induced at boundary B between proton exchange layers 3a and substrate 1 by thermal diffusion of H+ during the formation of polarization inversion layers. This method includes the steps of forming proton exchange layers 3a on the X-cut surface in such a manner that the extending direction of each proton exchange layer 3a is inclined at an angle of .phi. with respect to the Z-axis and then, performing a heat-treatment on the substrate to form deep polarization inversion layers on the X-cut surface. Although such a process for forming a proton exchange layer utilizing an internal electric field is effective for the fabrication of a low order QPM structure which has a large pitch of inversion polarization layers, for instance about 10 .mu.m, but not effective in a high order QPM structure which has a small pitch and width of proton exchange layers since internal electric fields cancel each other. As a result inversion polarization layers with a fine pitch are not fabricated.